The field of this invention relates to an apparatus for measuring linewidths on masks and wafers employed in the manufacture of large-scale-integrated (LSI) devices and more particularly, to an apparatus for accurately ascertaining the aforesaid linewidth measurements.
As is well known within the prior art, there are universally utilized various small-feature-size masks to fabricate microelement devices. A typical such mask comprises a planar glass member having an opaque coating such as chrome deposited thereon. In conventional ways, the opaque coating is selectively patterned to form multiple regions of chip areas, each having opaque and transparent portions. Illustratively, the regions are patterned to be identical replicas of each other. In each region, clear or transparent features may be formed in an opaque background, or vice versa.
Typically, the pattern or subregion, is visible to an operator using a microscope. By way of example, a typical subregion could be one millimeter in diameter and have an area of 0.8 square millimeter. Again, as herein above mentioned, the mask subregion comprises clear portions and opaque portions (or lines). It is well recognized that highly accurate control is required for the linewidths formed on masks and wafers as these linewidths are extremely critical to the overall process of manufacturing LSI devices. Thus, for example, before using a set of masks to form features on an associated wafer, it is important that each of the masks be checked to determine whether or not specified linewidths thereon have been made within specified tolerances. A wafer is an opaque base upon which lines are formed.
In a prior art linewidth measurement device, a procedure is followed which includes making a calibrated and normalized measurement of the average light transmission (or reflection) in a specified subregion, the latter including a feature whose linewidth is to be determined. In turn, this measurement is automatically converted to a linewidth reading by analog computing circuitry.
One known apparatus for measuring linewidths within selected subregions of a microelement, wherein each subregion includes features exhibiting two contrasting optical properties, comprises two reference subregions each uniformally exhibiting a different one of the two contrasting optical properties. Included within this apparatus is an illuminating source for directing light at each one of the reference subregions and at a selected subregion to be measured. Light responsive means is provided to obtain light directly from the source and from light which is impinged upon a reference subregion for generating normalized reference and subregion signals. A processor is arranged for the normalized signals to generate a signal representative of one of the optical properties of the selected subregion. Finally, a process is also arranged for the last mentioned representative signal in accordance with a predetermined relationship to generate a signal directly representative of a specified linewidth in a selected subregion. Although this type of apparatus has its own unique set of characteristics, it is complex in nature and calls for optical and electrical elements which are expensive to maintain and to operate.
Previous to this invention, all known linewidth measuring devices are moved lineally by moving the specimen. Moving of the specimen inherently requires tremendous accuracy and sensitivity in order to make accurate measurements. Accurate measurements are extremely necessary in order to successfully transfer a pattern from a mask. The measuring system used should be reliable in order to confidently proceed from one step to the next succeeding step in the forming of a microelement. The ultimate yield of usable chips per wafer is a direct function of each step in the process which includes the measuring of dimensions.
Normally a microelement is composed of a plurality of superimposed, thin wafer layers. Because each succeeding layer is superimposed on another layer, line dimensions become extremely important in order to achieve the desired electrical conducting path without the producing of any electrical shorts. Accordingly, there is a definite need for a simplified, low cost and accurate linewidth measurement apparatus.